1. Field of the Invention
The present invention relates to a cooling structure of an electronic equipment rack which comprises a plurality of shelf units stacked one upon another, each shelf unit including a plurality of printed circuit boards, for electronic equipment such as a communication equipment and an exchange, and particularly to a forced air cooling type cooling structure provided with fans.
2. Description of the Related Art
In electronic equipment such as communication equipment, a plurality of printed circuit boards are housed in a shelf unit having a predetermined function, and a plurality of the shelf units are stacked one upon another and arranged in a cabinet or fitted to a frame to constitute a rack having a particular function. The shelf units must be cooled for long term stabilization of the function of respective printed circuit boards housed in each shelf unit. Recently, the thermal density of a printed circuit board and of a shelf unit accommodating plural printed circuit boards have increased with the advance of a high density arrangement of elements on the printed circuit board and high-level integration of the individual elements. For example, the thermal density of a conventional printed circuit board is 10 watts per liter, as indicated by a black dot shown in FIG. 17. On the other hand, the technical development in recent years has brought the power consumption to 1/2 and the volume to 1/4 of those in the conventional printed circuit board, with the result of increasing the thermal density to 20 watts per liter, which is twice the conventional case, as indicated by a white dot in the figure.
To cool the shelf unit accommodating printed circuit boards exhibiting such a large thermal density, a forced air cooling type cooling structure provided with fans is used. In this type of cooling structure, an important factor for realizing effective heat radiation is not only the temperature of the cooling air which passes over the printed circuit board but also the velocity of the cooling air.
FIGS. 15 and 16 show a prior art cooling structure. In the figures, two shelf units 101 are stacked one upon the other, and a cooling unit 102 is disposed on top of them. The stacked shelf units 101 and cooling unit 102 constitute a cooling block, and a plurality (two in the example shown in FIG. 15) of the cooling blocks are stacked and received in a cabinet (not shown) to constitute a rack 105 as a whole. As shown in FIG. 16, a plurality of printed circuit boards 103 are housed side by side in each shelf unit 101. In the cooling unit 102, four fans (blowers) 104 each blowing frontward are provided. For the two-story, or two-tier, shelf units, the fan 104 takes atmosphere in from the bottom of the two-story shelf units, allows the air to pass through the shelf units, and discharges the air through a front door (not shown) of the cabinet at the front of rack 105, as indicated by an arrow A (FIG. 15).
The prior art cooling structure mentioned in the above has the following drawbacks:
(1) A plurality (two in this example) of the shelf units 101 stacked under the fan 104 are cooled by the cooling air which first cools the lower shelf unit and therefore, is heated by the lower shelf unit, and then cools the upper shelf unit by passing thereover. As a result, the temperature of the cooling air for cooling the upper shelf unit is higher than that for cooling the lower shelf unit, causing a temperature difference between the upper and lower shelf units. Accordingly, the cooling capacity for the upper shelf unit is lowered. Since an allowable total heating value in each cooling block is set according to the temperature of the shelf unit which will have a higher temperature (normally, the lower shelf unit), the heating value of the upper shelf unit will be restricted by the temperature difference mentioned above.
(2) If cooling units with fans are provided for respective shelf units to solve the problem mentioned above in (1), the cooling units will occupy a large space in the cabinet and prevent an overall high density arrangement in the cabinet.
(3) A flow of the cooling wind (arrow A) is suddenly changed from a bottom-to-top direction to a lateral (horizontal) direction by the rear, slope surface 102a of the cooling unit 102, and thus the flow resistance become large and the cooling efficiency is deteriorated.
(4) If a fan in the cooling unit 102 fails, the temperature of the shelf unit located under the cooling unit accommodating the failed fan tends to be increased, and reliability can not be attained.
(5) Since the cooling wind is discharged laterally (horizontally) through the front door of cabinet, the discharged air will give an uncomfortable feeling to maintenance personnel and a loud noise from the fans.